1. The Field of the Invention
The present invention relates to crystallizing glass solders and composites, which are suitable in particular for high-temperature applications, and to applications thereof.
2. The Description of the Related Art
Glass solders are usually used for producing joints to connect, in particular, glass and/or ceramic components to one another, or to components made of metal. In the development of glass solders, the composition thereof is often selected so that the coefficient of thermal expansion of the glass solder corresponds approximately to that of the components to be connected to one another in order to obtain a joint which is stable in the long term. Compared to other joints, for example those composed of plastic, those based on glass solders have the advantage that they can produce a hermetic seal and can withstand relatively high temperatures.
Glass solders are generally often produced from a glass powder which is melted during the soldering operation and, together with the components to be connected, forms the joint under the action of heat. The soldering temperature is generally selected so as to correspond approximately to the hemisphere temperature of the glass or can usually deviate from the latter by ±20 K. The hemisphere temperature can be determined in a microscopic method using a hot stage microscope. It characterizes the temperature at which an originally cylindrical test specimen has fused together to form a hemispherical mass. The hemisphere temperature can be assigned a viscosity of about log η=4.6, as can be seen from the relevant technical literature. If a crystallization-free glass in the form of a glass powder is melted and cooled again so that it solidifies, it can usually also be re-melted at the same melting point. In the case of a joint comprising a crystallization-free glass solder, this means that the operating temperature to which the joint can be subjected in the long term must be no higher than the soldering temperature. In actual fact, the operating temperature in many applications has to be significantly below the soldering temperature since the viscosity of the glass solder decreases with increasing temperatures and a glass having a certain flowability may be forced out of the joint at high temperatures and pressures, with the result that the joint may fail.
For this reason, non-crystallizing glass solders for high-temperature applications must usually have a soldering temperature or hemisphere temperature that is even significantly above the later operating temperature. One problem which may arise as a result of the much higher soldering temperature in comparison with the later operating temperature is the damage to the components to be connected to one another. Therefore, glass solders which, though they have as low a soldering temperature as possible, nevertheless allow an operating temperature that is as high as possible are desired. This means that, after a first soldering operation, the desired glass solders should only be re-meltable at a higher temperature than the soldering temperature.
This cannot be readily achieved with non-crystallizing glass solders as such. Glass solders that meet such requirements can be obtained, however, if the base glass at least partially crystallizes during the soldering operation, wherein the crystalline phases may have properties that deviate significantly from the base glass, for example with respect to the thermal expansion, but, in particular, the temperature required for the re-melting generally should be significantly above that of the base glass. The properties of an at least partially crystallized glass solder can be influenced directly by the composition of the original base glass, but also by suitable fillers, which generally have a crystalline structure and are added to the solder glass. The mixture of glass solder and filler is referred to in this disclosure as a composite.
The crystallization properties of the glass solder and/or composite are of outstanding importance for the processing properties when creating the joint and for the durability of the joint. The joining process generally comprises heating up to a joining temperature, introducing the glass solder at the joining temperature into the site of the joint and a crystallization phase in which the workpiece and glass solder are kept at a crystallization temperature below the joining temperature. The glass solder should optimally not yet crystallize during the heating, or only slowly, since it otherwise does not wet very well the surfaces to be joined, which would have to be compensated by a much higher joining temperature. Metallic elements involved in the joint could undergo undesired oxidation reactions as a result of increased joining temperatures. A resultant oxide film of a certain thickness may already peel off during the soldering operation and thus prevent a sealed connection. Furthermore, at such high soldering temperatures, there is increased vaporization of Cr from steels which often constitute an element of the components of the joint. During the cooling down of the temperature in the crystallization phase, a crystallization should take place as quickly as possible, but optimally the glass solder should not be completely crystallized but retain an amorphous, vitreous phase. This residual glass phase prevents brittle characteristics of the glass solder and can even contribute to healing cracks in the joint.
One field of use of such glass solders and/or composites is, for example, that of joints in high-temperature fuel cells, which can be used for example as an energy source in motor vehicles. An important type of fuel cell is, for example, the SOFC (solid oxide fuel cell), which can have very high operating temperatures of up to approximately 1000° C. The joint comprising the glass solder is usually used for producing fuel cell stacks, i.e. for connecting a plurality of individual fuel cells to form a stack. Such fuel cells are already known and are continually being improved. In particular, the trend in present-day fuel cell development is generally in the direction of lower operating temperatures. Some fuel cells already achieve operating temperatures below 800° C., so that a lowering of the soldering temperatures is possible and also desirable because of the resulting low thermal stress of the SOFC components during the soldering process.
Apart from the high-temperature resistance and the processing properties of the glass solders, a low electrical conductivity of the glass solders is required, for example in the high-temperature fuel cell, which generally requires alkali-free solders.
Furthermore, in this area there is the requirement that the solders must be free from substances such as Pb, a very commonly used constituent of glass solders with a great influence on crystallization and processing properties.
DE 19857057 C1 describes an alkali-free glass-ceramic solder having a coefficient of thermal expansion α(20-950) of from 10.0·10−6 K−1 to 12.4·10−6 K−1. The solder described there contains from 20 to 50 mol % of MgO. Glasses having a high MgO content are in practice highly susceptible to crystallization, which leads to compounds which crystallize rapidly and to a high degree. In the case of such rapid and substantial crystallization, it is difficult to ensure good wetting of the interconnecting material by the glass solder. However, this is necessary to be able to provide a joint which optimally satisfies the respective requirements.
Glass-ceramic solders are also described in U.S. Pat. No. 6,532,769 B1 and U.S. Pat. No. 6,430,966 B1. These are designed for soldering temperatures of approximately 1150° C. and contain from 5 to 15 mol % Al2O3. Such high soldering temperatures are undesirable for modern fuel cells, since they subject the metallic substrate materials and other temperature-sensitive materials to excessive loads.
DE 10 2005 002 435 A1 describes composite solders which consist of an amorphous glass matrix and a crystalline phase. However the glass matrix has a high content of CaO, which leads to relatively high viscosities, very great crystallization and high dielectric losses.
Glass and glass-ceramic sealing materials are commonly used in the case of planar high-temperature fuel cells, since these materials can be readily adapted in terms of their coefficients of thermal expansion and viscosities to the conditions and sealing partners. The high demands placed on the materials (high operating temperatures of up to 850° C., moist fuel gases, thermocycles) mean that materials which can satisfy these demands are required. The materials that are primarily used crystallize virtually completely at the operating temperatures. The crystallization then leads to undesirable changes to the properties in terms of the porosity and thermal expansion properties. These materials can often only undergo insufficient thermocycling and the fluctuating temperature loads lead to cracks in the microstructure. Another disadvantage is the often inadequate resistance of the glasses in a moist fuel gas atmosphere; particularly when the glasses have a very high boron content, or when the materials do not undergo sufficient “dense sintering” because of the premature crystallization.
Within this disclosure, the term “crystallizing glass solder” comprises glass solders which at least partially crystallize during the soldering process, or preferably in a subsequent process, while amorphous, vitreous phases may also still be present in the glass solder. Correspondingly, the state of the glass solders after processing is referred to as crystallized, even if amorphous, vitreous phases may still be present in the glass solder.